1. Field of the Invention
The present invention relates to a process of and a system for producing a high-concentration low rank coal-water slurry, and further relates to a slurry transfer mechanism included in the system.
2. Description of the Prior Art
Coal-water slurries are produced by adding water and additives to coal powder obtained by finely grinding coal. Since the coal-water slurry is in the form of fluid, handling thereof is easy. Further, the price of the high-concentration coal-water slurry per unit calorie is lower than a heavy oil or the like. Accordingly, attention has been paid thereto as a fuel replacing petroleum. The coal-water slurry is required to have a high concentration of 60 to 70 weight % of coal for good thermal decomposition and gasification and further for high transportation efficiency. If low grade coal, such as sub-bituminous coal or lignite, is used as a material of the coal-water slurry, since the low grade coal is highly hygroscopic and highly moist and includes lots of oxygen-containing hydrophilic groups, such as phenol or carboxyl groups, and thus is high in hydrophilicity on the surface thereof, it has been not easy to produce the high-concentration coal-water slurry.
Under these circumstances, technology has been proposed for improving the quality of the low grade coal to achieve the high productivity of the high-concentration coal-water slurry. For example, Japanese Second (examined) Patent Publication No. 5-76993 discloses a technique, wherein the low grade coal is heated to 180 to 450.degree. C. using a high-temperature gas so as to be improved in quality, and then the improved coal is ground and mixed with water at a given concentration to be formed into a coal-water slurry. Japanese First (unexamined) Patent Publication No. 52-71506 discloses a technique, wherein the quality of solid fuel is improved under a pressurized hydrothermal (hot water) atmosphere at 300 to 700.degree. F. and after the quality improvement, the improved fuel is adjusted to a given particle size distribution to obtain a slurry. Japanese First (examined) Patent Publication No. 60-152597 discloses a technique for accomplishing further quality improvement using additives as an example of quality improvement in a non-vaporization dehydrating process.
However, the present inventors have found that any of the foregoing conventional techniques can not achieve the high improvement in quality and thus is not sufficient for producing the high-concentration coal-water slurry. Further, no attention has been paid to the effective utilization of waste water generated upon production of the coal-water slurry, which, hence, still remains as an outstanding problem.
On the other hand, in the course of producing the coal-water slurry, it is necessary that a high-pressure slurry is transferred from a high-pressure slurry vessel to a low-pressure slurry vessel through a valve while reducing a pressure of the high-pressure slurry. However, since a pressure differential between the high-pressure slurry vessel and the low-pressure slurry vessel is large, a pressure drop generated at the valve is also large. Thus, the flow velocity of the slurry is high upon passing the valve to cause abrasion or erosion of the valve. If vaporization occurs upon pressure reduction, the erosion becomes more intense.
If the valve is subjected to abrasion to a certain degree, it may be necessary to exchange a worn part of the valve which is high-priced in general. Further, it takes much time for valve maintenance including an exchanging operation for the worn part.